Chimpanzees process structural isomorphisms across sensory modalities.

Evolution has shaped animal brains to detect sensory regularities in environmental stimuli. In addition, many species map one-dimensional quantities across sensory modalities, such as conspecific faces to voices, or high-pitched sounds to bright light. If basic patterns like repetitions and identities are frequently perceived in different sensory modalities, it could be advantageous to detect cross-modal isomorphisms, i.e. develop modality-independent representations of structural features, exploitable in visual, tactile, and auditory processing. While cross-modal mappings are common in the animal kingdom, the ability to map similar (isomorphic) structures across domains has been demonstrated in humans but no other animals. We tested cross-modal isomorphisms in two chimpanzees (Pan troglodytes). Individuals were previously trained to choose structurally 'symmetric' image sequences (two identical geometrical shapes separated by a different shape) presented beside 'edge' sequences (two identical shapes preceded or followed by a different one). Here, with no additional training, the choice between symmetric and edge visual sequences was preceded by playback of three concatenated sounds, which could be symmetric (mimicking the symmetric structure of reinforced images) or edge. The chimpanzees spontaneously detected a visual-auditory isomorphism. Response latencies in choosing symmetric sequences were shorter when presented with (structurally isomorphic) symmetric, rather than edge, sound triplets: The auditory stimuli interfered, based on their structural properties, with processing of the learnt visual rule. Crucially, the animals had neither been exposed to the acoustic sequences before the experiment, nor were they trained to associate sounds to images. Our result provides the first evidence of structure processing across modalities in a non-human species. It suggests that basic cross-modal abstraction capacities transcend linguistic abilities and might involve evolutionary ancient neural mechanisms.